<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <title>Document</title>
  </head>
  <body>
    <script>
      var num = prompt('请输入星星的个数');
      var str = '';
      for (var i = 1; i <= num; i++) {
        str = str + '★';
      }
      console.log(str);
    </script>
  </body>
</html>